This is a nationalization of PCT/JP00/04600 filed Jul. 10, 2000 and published in Japanese.
The present invention relates to a magnetic fluid seal that seals various fluids using a magnetic fluid. For instance, it relates to a magnetic fluid seal that is applied as a shaft sealing apparatus used for a semiconductor manufacturing process, a vacuum process, various pneumatic devices or the like.
As a reciprocating seal for sealing a circular space between a shaft and a housing that reciprocate relatively, there has conventionally been used an elastic seal that is made of an elastic material such as rubber.
In the case where such an elastic seal is used, the elastic seal seals a target member that should be sealed while contacting the target member, and thus there occur problems concerning slide resistance and friction due to sliding motion.
On the other hand, there has been known a magnetic fluid seal as a seal that achieves a sealing effect in a non-contact manner, which does not cause the stated problems.
Magnetic fluid seals according to conventional techniques will be described with reference to FIGS. 5 and 6.
First, a magnetic fluid seal 100 shown in FIG. 5 is provided to seal a circular space between a shaft 300 and a housing 400.
This magnetic fluid seal 100 substantially includes: a circular permanent magnet 101 that is arranged so that one end thereof corresponds to a north pole and the other end thereof corresponds to a south pole in a shaft extending direction; and a pair of pole pieces 102 and 103 that are respectively provided on both sides so that the permanent magnet 101 is sandwiched therebetween.
Here, the housing 400 shown in FIG. 5 is made of a non-magnetic material and a space between the magnetic fluid seal 100 and the interior surface of the housing 400 is sealed with O rings D10 and D20.
On the other hand, the shaft 300 is made of a magnetic material and a space between the magnetic fluid seal 100 and the surface of the shaft 300 is sealed with a magnetic fluid J.
That is, there are formed a plurality of projections 102a, 102b, and 102c on the pole piece 102 and there are also formed a plurality of projections 103a, 103b, and 103c on the pole piece 103. The magnetic fluid J is held by a magnetic force in a space between the tips of these projections and the surface of the shaft 300, thereby achieving a sealing effect.
Also, in the same manner, a magnetic fluid seal 200 shown in FIG. 6 is provided to seal a circular space between a shaft 301 and a housing 401.
Like the magnetic fluid seal 100 shown in FIG. 5 described above, this magnetic fluid seal 200 also includes a permanent magnet 201 and a pair of pole pieces 202 and 203.
Here, the shaft 301 shown in FIG. 6 is made of a non-magnetic material and a space between the magnetic fluid seal 200 and the surface of the shaft 301 is sealed with O rings D30 and D40.
On the other hand, the housing 401 is made of a magnetic material and a space between the magnetic fluid seal 200 and the interior surface of the housing 401 is sealed with a magnetic fluid J.
That is, there are formed a plurality of projections 202a, 202b, and 202c on the pole piece 202 and there are also formed a plurality of projections 203a, 203b, and 203c on the pole piece 203. The magnetic fluid J is held by a magnetic force in a space between the tips of these projections and the interior surface of the housing 401, thereby achieving a sealing effect.
For instance, Japanese Utility Model Application Laid-Open No. Sho 55-20746 discloses a technique with which, between a rotary shaft and a fixed casing, a pair of circular ball blocks is fixed to bearings for holding the rotary shaft, a magnet is also provided between these paired circular ball blocks, and a magnetic fluid is held by this magnetic force between the rotary shaft and projections formed at tips of the circular ball blocks. In this manner, there is achieved a construction in which the rotary shaft is sealed with this magnetic fluid.
Also, Japanese Utility Model Examined Publication No. Sho 63-11427 discloses a technique with which, in a space between a casing and a rotary shaft supported by bearings within the casing, a magnetic fluid is held between the rotary shaft and projections formed on the surfaces of bushes that are fixed to the rotary shaft so that a magnet is sandwiched between these bushes and the rotary shaft. In this manner, there is achieved a construction in which a space between the rotary shaft and the projections formed on the bush surfaces is sealed with the magnetic fluid.
In the cases of the aforementioned conventional techniques, however, there occurs a problem to be described below.
As described above, in the case where a magnetic fluid seal is used, there occur no problem concerning slide resistance and friction due to sliding motion.
However, in the case of a magnetic fluid seal, there occurs a problem that a held magnetic fluid leaks, losing its sealing property.
This problem in which a magnetic fluid leaks rarely becomes serious in the case where a magnetic fluid is applied as a rotary seal for sealing a space between two members that rotate relatively to each other. However, in the aforementioned case where a magnetic fluid is applied as a reciprocating seal for sealing a circular space between a shaft and a housing that reciprocate relatively, it is difficult to prevent leakage of the magnetic fluid. This makes it difficult to apply a magnetic fluid to a reciprocating seal.
This point will be described in more detail by taking, as an example, the magnetic fluid seal shown in FIG. 5 or 6 described above.
The length in the shaft extending direction of each of the permanent magnets 101 and 201 that are respectively shown in FIGS. 5 and 6 is generally set to about 5 mm or longer so as to secure a magnetic force.
Accordingly, in the case of the magnetic fluid seal 100 shown in FIG. 5, there is maintained a distance having a substantially similar length between the projections 102a and 103a of the respective pole pieces 102 and 103 that are provided so that the permanent magnet 101 is sandwiched therebetween. As a result, there is formed a space indicated by reference symbol P in the drawing.
Also, in the same manner, in the case of the magnetic fluid seal 200 shown in FIG. 6, there is maintained a distance having a substantially similar length between the projections 202a and 203a of the respective pole pieces 202 and 203 that are provided so that the permanent magnet 201 is sandwiched therebetween. As a result, there is formed a space indicated by reference symbol Q in the drawing.
Meanwhile, the surfaces of magnetic materials forming these P space and Q space are areas having the strongest magnetic fields.
Accordingly, there are cases where a magnetic fluid that should be held by the projections 102a and 103a or by the projections 202a and 203a leaks to the surface of the magnetic material forming the P space or the Q space. As a result, it becomes impossible to secure a sealing property.
Also, in the case where the moving speeds of a shaft and a housing that reciprocate relatively are high, there may be a case where a magnetic fluid leaks outside of a magnetic fluid seal.
The present invention has been made to solve the aforementioned problem of the conventional techniques, and the object of the present invention is to provide a magnetic fluid seal, which reduces leakage of a magnetic fluid with an excellent quality.
To achieve the above-mentioned object, according to the present invention, a magnetic fluid seal for sealing a circular space between a shaft and a housing, comprises: a magnetic force generating member that is arranged so that one end thereof corresponds to a north pole and the other end thereof corresponds to a south pole in a shaft extending direction; and a pair of magnetic pole members that are respectively provided on both sides so that the magnetic force generating member is sandwiched therebetween, wherein the magnetic fluid seal is characterized in that each of the pair of magnetic pole members includes a plurality of holding projections that extend toward a surface of the shaft and hold a magnetic fluid in a space between the holding projections and the shaft surface, and a distance between holding projections, which are nearest to the magnetic force generating member among the holding projections of the pair of magnetic pole members, is shorter than a length of the magnetic force generating member in the shaft extending direction.
Accordingly, the magnetic fluid is held at positions having strong magnetic field that are placed inward from the positions between which there exists a distance equal to the length of the magnetic force generating member. As a result, it is possible to suppress leakage of the magnetic fluid.
It is preferable that an intercepting seal for intercepting a space between the magnetic force generating member and the shaft surface is provided between the holding projections that are nearest to the magnetic force generating member.
With this construction, it is possible to prevent a situation where the magnetic fluid leaks toward the magnetic force generating member side.
It is preferable that the holding projections, which are farthest from the magnetic force generating member among the holding projections of the magnetic pole members, are provided at positions inward from end surfaces in the shaft extending direction of main bodies of the magnetic pole members, and a distance between the shaft surface and wall surfaces, which oppose the shaft surface and exist at positions outward from the holding projections that are farthest from the magnetic force generating member, is longer than a distance from tips of the holding projections to the shaft surface and also is shorter than the heights of the holding projections.
With this construction, it is possible to suppress a situation where the magnetic fluid held by the holding projections that are farthest from the magnetic force generating member leaks to the outside.
It is preferable that the wall surfaces opposing the shaft surface are each formed as a tapered surface whose distance to the shaft surface is gradually reduced as directing from an outer side to an inner side.
With this construction, it is possible to further suppress the situation where the magnetic fluid held by the holding projections that are farthest from the magnetic force generating member leaks to the outside.
A magnetic fluid seal for sealing a circular space between a shaft and a housing, comprising: a magnetic force generating member that is arranged so that one end thereof corresponds to a north pole and the other end thereof corresponds to a south pole in a shaft extending direction; and a pair of magnetic pole members that are respectively provided on both sides so that the magnetic force generating member is sandwiched therebetween, wherein the magnetic fluid seal is characterized in that each of the pair of magnetic pole members includes a plurality of holding projections that extend toward a surface of the housing and hold a magnetic fluid in a space between the holding projections and the housing surface, and a distance between the holding projections, which are nearest to the magnetic force generating member among the holding projections of the pair of magnetic pole members, is shorter than a length of the magnetic force generating member in the shaft extending direction.
Accordingly, the magnetic fluid is held at positions having a strong magnetic field placed inward from positions between which there exists a distance equal to the length of the magnetic force generating member. As a result, it is possible to suppress the leakage of the magnetic fluid.
Here, it is preferable that an intercepting seal for intercepting a space between the magnetic force generating member and the housing interior surface is provided between the holding projections that are nearest to the magnetic force generating member.
With this construction, it is possible to prevent a situation where the magnetic fluid leaks toward the magnetic force generating member side.
It is preferable that the holding projections, which are farthest from the magnetic force generating member among the holding projections of the magnetic pole members, are provided at positions inward from end surfaces in the shaft extending direction of main bodies of the magnetic pole members, and a distance between the housing interior surface and wall surfaces, which oppose the housing surface and exist at positions outward from the holding projections that are farthest from the magnetic force generating member, is longer than a distance from the tips of the holding projections to the housing inner peripheral surface and is also shorter than heights of the holding projections.
With this construction, it is possible to suppress a situation where the magnetic fluid held by the holding projections that are farthest from the magnetic force generating member leaks to the outside.
It is preferable that the wall surfaces opposing the housing interior surface are each formed as a tapered surface whose distance to the housing interior surface is gradually reduced as directing from an outer side to an inner side.
With this construction, it is possible to further suppress the situation where the magnetic fluid held by the holding projections that are farthest from the magnetic force generating member leaks to the outside.
It is preferable that a taper angle of each of the wall surfaces opposing the housing interior surface is set at 45xc2x0 or less. Also, it is further preferable that the wall surfaces opposing the housing interior surface are each formed so that the taper angle is set in a range of from 5xc2x0 to 30xc2x0.
It is preferable that the distance between the holding projections that are nearest to the magnetic force generating member is set in a range of from 0.1 to 2 mm. With this construction, it is possible to effectively prevent leakage of the magnetic fluid.